Archives For RaCy

Before we get to Katie, it’s worth mentioning recent and current weather for Easter Sunday: the first convective day of 2016. A cold pool of polar upper air followed the impressive cold front yesterday which developed a very long squall line.

squall line cold front

cold upper air 500hPa

cold front moving east

Thundery conditions followed the cold front in an unstable Polar airmass with notable CAPE (convective available energy) and negative 2 Lifted Index for the time of year – both indicators of potential thundery heavy shower activity. The limiting factor on multicell thunderstorms locally was lack of wind shear and an anticyclonic jetstream, both subduing anything truly impressive or sustained. Nevertheless, some hail and thunderstorm activity is rife across the UK today in this unstable polar maritime airmass.

Storm Katie is riding a powerful jetstream and deepening rapidly over the Atlantic through today. She is some 600miles away to the SW but approaching and deepening rapidly as she undergoes rapid cyclogenesis (RaCy) falling from 999mb to 975mb: 24mb in 24 hours, qualifying as a “bomb depression”.

Katie emerged out of Canada earlier this week as cold continental air met humid sub-tropical maritime air. She engaged with an active jetstream to be launched across the Atlantic. Storms tend not to move fast and deepen rapidly at the same time, so her rapid progress across the Atlantic will now slow as she now expends more energy lowering pressure on her approach to the UK.

The evolution and track of this storm is similar to St Jude October 2013, but Katie is forecast to be less powerful, due in part to the slacker pressure rise on her departure into the North Sea.** (update: in the event Katie pipped StJude, see below!)

St Jude fax chart October 2013

October 1987 storm fax chart

Katie also has some similarities to the October 1987 but again is significantly less powerful than that rare 200 year event.

The water vapour sat pics below show the evolution of Katie and cirrus cloud associated with the jetstream.

Storm Katie is forecast to deepen through the SW approaches to below 980mb on an expected track overnight Sunday-Monday from Biscay into the Bristol Channel through to exit around the Wash.

The pressure gradient between the Isle of Wight and the Wash could be as much as 12mb… the highest of our Wight-Wash Oscillations recorded, and greater than the 10mb WWO recorded in StJude, if this comes off.

However, the pressure rise behind Katie is expected to be less rapid and sustained than St Jude, so this critically reduces the potential for highest gusts, sting jets aside.

The highest wind speeds for Storm Katie will be south of the low core in the occluding warm sector in the small hours of Monday morning.

Max gusts of over 80mph are possible in the Channel, 70mph along Channel coasts and 60mph further inland. For Reigate this means a brief episode of gusts possibly exceeding 50mph and more over exposed hills. This could wake people up momentarily as gusts roar through trees and round houses.

Rainfall will be significant too and some hi res models put down >40mm over parts of the North Downs, showing the significance of small hills in generating orographic rainfall.

Rainfall more widely is more likely to be around 20-30mm, enough for local flooding. The MetOffice have issued a yellow warning for Storm Katie. Check the weather impact matrix showing moderate likelihood of moderate impact (low yellow).

Update: Amber warning issued for Storm Katie.

The shipping forecast is less equivocal and shows storm warnings right around the UK associated with Katie and other low pressure systems. Storm Force 10 is possible in the Channel for a while. http://bbc.in/1RtwvyS

Furthermore, evolution so far matching StJude and the Wight-Wash pressure gradient forecast seems to be matching or beating St Jude.

Below is a Meteosat picture (courtesy Dundee sat receiving station) showing Katie moving into the North Sea, deepening again slightly to 972mb,

Meteosat visible photo of Storm Katie moving into North Sea on Monday 28 March

Post-Katie we enter a broadly zonal westerly flow but turning cooler as slack winds swing northerly through the week as LOW pressure moves into North Sea/Scandinavia. Unsettled and showery activity continues through the week but with pressure slowly rising from the west by Thursday. No further significant storms are expected for the time being. Into the start of April, pressure over the Atlantic is expected to rise and progress over the south of the UK into Europe to give a settled period for the start of April.

RGSweather is off to East Iceland, so expect occasional tweets and updates from cooler climes.

Storm Katie: In the Event:

various meteorological impacts show Katie exceed StJude in power but both fall well short of October 1987 Great Storm.

Note, compared with the Oct ’87 storm Katie and St Jude are found wanting. The Wight-Wash Oscillation for the 200 year storm of October 1987 was a whopping 20mb! See the chart below for the exit of the Oct 87 storm through the Wash on 16 October 1987.

So far this winter Reigate and the SE has been sheltered from much of the weather action, which has focused mainly on the NW and especially Scotland. This coming week might change that somewhat, albeit modestly in comparison with the battering the NW has received. Charts below show temperatures and pressure are overall on the slide this week while precipitation makes two noticeable spikes.

frontal systems, then a storm,

cooler overall

UKstorm: dip in pressure later this week

updated 12/01

A moist warm airflow ahead of a slow moving cold front arriving from the north Monday pm could stall over the SE overnight into Tuesday am and bring significant rainfall totals. Temperatures Monday night could hold up into double figures as a warm moist SW flow funnels up from the subtropics ahead of the sluggish cold front.

slow cold front clearing south

The wave on the polar front emerges out of the SW through Monday with a characteristic plume of rain and a dip in pressure. Such waves tend to bring a lot of rain despite hardly showing up on synoptic charts. A modest kink in the front and isobars (see charts below) is the only hint of potential heavy rain action. Some wintry precipitation and maybe snow is possible for places on the north side of the front later into Tuesday, in Wales/Midlands for example, but not for the SE as temperatures remain too high. The front slips slowly south Mon/Tues bringing some significant rain and then brisk cooler weather from showery westerly winds to follow, some sparse showers could possibly be wintry Tues-Weds but not amounting to much for us, most wintry showers will fall further west. Temperatures will be cooler at Tmax 5-6 on Tuesday and even cooler on Weds, so cold but frost unlikely as too breezy.

#1 wave depression

#2 wave depression

slow mo cold front

high totals Mon-Tues

There is a weather warning out for this period especially for the south coast but Reigate totals could reach 20mm+ and exceed 20-30mm for places nearer the south coast in Sussex. The front is followed by increasingly cool showery westerly winds through Tuesday into Wednesday with the odd wintry shower possible but nothing significant.

synoptic situation

classic bomb depression

produce hurricane force at sea

60mph+ ?

left exit region of jet

There is the possibility of a storm for the UK Wednesday-Thursday bringing gales to our area as a “bomb depression” emerges in the Atlantic this week and probably tracks across Scotland (which would be less windy as a result, for a change). Details of the storm track remain wobbly at the moment but the strongest winds will be found on the southern flank of the LOW centre and various models suggest that the SE could have some strong to gale force winds, albeit nothing like the scale of Scotland recently. Models suggest a steep pressure gradient as the low tracks somewhere across the north of the UK. The precise track will determine the impact on the SE. Current models suggest the strongest winds for the SE will be associated the warm front approaching Wednesday night / Thursday morning. Gusts could exceed 50mph inland and more on the coast with some excitable models suggesting more but these usually calm down nearer the time! Nevertheless, wind and rain are likely to be significant and expect updates on weather warnings this week. Colder air follows the storm so showers following cold fronts could turn wintry so watch for updates.

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Our own amateur “Wight-Wash Oscillation (WWO)” is an attempt to quantify the potential impact / severity of storms on the South East of England. The WWO is simply a crude measure of pressure difference between the Wash and the Isle of Wight ( a mini NAO!). As a benchmark the St Jude storm (99mph Needles max gust) had a WWO of approx 10mb (976 Wash-986mb Wight) and the modest storm of 12/12/14 was a 9mb on the WWO.

Current models shown below have a variety of WWO results from 8mb UMET up to 12mb for GFSP. A difference of 1 or 2mb can make a big difference to the severity of the storm as pressure gradient ultimately drives wind speed. Nevertheless, the WWO is just for fun but interesting to see how it works out in the event. On the basis of our WWO this storm currently looks less severe than St Jude, which is a good thing because that storm caused a reasonable amount of damage.

GFSP 12mb WWO

UKMET 8mb WWO

ensemble 8mb WWO

The unsettled weather this week is being driven by an active jetstream over the Atlantic. This itself is partly a product of the tremendous temperature gradient over the northern hemisphere and particularly over the North Atlantic with freezing air pouring out of a frigid Canada meeting warm sub-tropical air emerging round the Azores High from the south. These air masses are set to clash on Monday at the polar front over the Atlantic.

produce hurricane force at sea

clash of clans over ocean

Whatever the storm brings it is looking likely that afterwards a cool northerly wind from the Arctic will bring temperatures further down by the weekend. Although a long way off, for Reigate and the SE it is unlikely to be snowy and most likely to be the frosty and cool type of cold! Further ahead some models build a pattern into later January (around 20) that could be conducive to cooler conditions: with possible easterly or NE winds for a time as a low sinks into Europe, though this is not looking either particularly extreme or long-lived.

cool off post storm

cooler winds from N/NW

spot the LOW tucked in S: easterlies

cool Arctic blast next weekend poss

Meanwhile, the North Atlantic Oscillation is set to remain positive, though decline somewhat to neutral, which means that the LOW over Iceland and HIGH over the Azores synoptic pattern is likely to be maintained, albeit possibly less pronounced and with more anomalous meridional moments of cold incursions possible. This would mean a continuation of the broadly zonal westerly flow of mild windy and fairly wet unsettled weather for the foreseeable future. Conflicting signals like this have riddled longer range forecasters this winter: the stratospheric warming was only moderate, much to snow-lovers disappointment, and some supposed good indcators of colder winters (the OPI / early Siberian snow cover in October) have apparently failed to live up to forecasting skill expectations as yet. A month or so of winter to go and no significant signs of persistent cold … yet!

The MetOffice charts above show the formation and life-cycle of a December 2014 “weather bomb”, involving the process more properly called rapid cyclogenesis. There are reasons why meteorologists dislike the term “weather bomb” but perhaps the most obvious is that the sensationalist short-hand use of the term “bomb” detracts from the complex processes and variable scale and location of impacts. The term “bomb” tends to hype stories in the press that can cause over-reaction and unnecessary concern. On the other hand it gets people reading about the weather, which is a good thing (like this post, ahem!).

feed twitter hype

outrageously inaccurate

then discussion over weather semantics

some high winds

most impacts in Scotland

accurate metoffice warnings

Nevertheless, a “weather bomb”, a term borrowed from the US and New Zealand, is short-hand for a potentially extreme event. Bomb depressions are deep low pressure systems that form by the process of Rapid Cyclogenesis (RaCy for short). RaCy is the rapid formation of a deep depression when the central pressure falls more than 24mb in 24 hours. Such RaCy depressions are usually of marine origin. About 12 such RaCy bomb depressions hit the UK in the exceptionally stormy winter last year 2013. Although by no means the most powerful, the first and most famous RaCy depression of last winter was the St Jude storm that hit Southern England with moderate force in October 2013. Pictures below are from that event and can be compared to the enormous scale of the more recent Atlantic bomb depression of December 2014.

St Jude storm 2013 formation satellite pic

cooler polar airmass

synoptic chart of UKstorm 27-28 October 2013

Meteosat 15:00hrs and features

The “bomb” depression that struck this December 2014 seemed to catch media attention, despite the impressive weather impacts being almost wholly restricted to the less populated NW, especially Scotland, where people are entirely used to coping with such lively weather.

The December 2014 “weather bomb” was a depression (low pressure system) which formed rapidly far out west in the Atlantic between SE Greenland and Iceland. The formation was associated with a fast moving jetstream and the surface convergence of sub-tropical air from the south west meeting a frigid NW polar airstream from Canada and more local air direct from the Greenland ice cap. The big temperature differences between these air masses accelerated uplift and the lowering of central pressure.

airmass: reds are polar, blue/green tropical!

lively 200mph jetstream

1# isotachs in m/s shows jetstream core

RaCy Atlantic chart Dec 2014

impressive but not the day after tomorrow

Descending dry stratospheric air is another defining feature of RaCy systems. Cold dry air from aloft turbo-charges the depression as it is injected into the depression. The cold air aloft increases lapse rates in the surface airmass and causes air to rise more purposefully creating a dramatic fall in central pressure. Descending cold dry stratospheric air can be spotted on the water vapour satellite images as a dark dry slot ingressing into the depression circulation over time and following hard on the heels of the cold front as it is blasted across the Atlantic. The water vapour images below show the rapid development of the system during Tuesday 8 December. In later images it is possible to see the speckly cumulonimbus clouds emerging in the unstable cold sector following the cold front. Such instability was caused by the descending dry air.

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Rather than going through the rather measured development stages of a Norwegian Model cyclone, a RaCy depression usually follows a life cycle more like the Shapiro-Keyser model below (though at the time of writing I am not certain as to whether the December 2014 RaCy depression formally fitted all aspects of this model). Several key characteristics of the December 8 cyclone fit the S-K model fit and this is the usual model associated with RaCy depressions.

The Shapiro-Keyser depression life-cycle model often features a cold front that is blasted rapidly ahead. so rapidly that it “fractures” from the wrapping warm front further north. This is known as a T-bone fracture and experts can identify the moment of fracture using satellite photos. Additionally, cf course, upper air moves faster than the surface wind that suffers frictional drag even across relatively smooth ocean.

satellite features of emerging RaCY depression

This meant that the cold front moved so rapidly that it split vertically into a fast moving upper front and a slower moving surface cold front. The cold front literally had its head ripped off! The frigid upper cold air travelled over a shallow moist zone of warmer sub-tropical air and it is this that increased lapse rates and caused immense instability in the polar air stream that eventually arrived in Scotland. Instability can be seen on the visible satellite pics as speckly masses of cumulonimbus clouds shown best in the satpic above. In the charts and sat pics below note the wind speed associated with this polar air and the tropical air preceding it in the warm sector.

unstable polar air reaches Scotland

WV satellite image 9/12/14

airmasses: red=polar, blue/green=tropical!

wind speed, spot the polar airmass

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In the S-K model the cold front is sometimes weakened during the formation process while the warm front remains active, wrapping itself in knots around the central “eye” of the storm. The 850mb chart below shows temperatures of this cold upper air at 1500m above Scotland. The bomb depression this December seems to have matched this because, while the cold front was relatively weak (narrow squall line) the exceptionally unstable polar air behind it was arguably the defining characteristic of this system, bringing persistent convective storms and an outstanding 5000 lightning strikes and thunder-snow blizzards across higher ground in Scotland during the advection of this exceptionally cold and unstable air for an Atlantic NW airstream.

frigid 850hPa upper air temps

surface sea level pressure

thunderstorms and snow over Scotland

storm force winds

60ft waves

Scotland left snowy

In the S-K model depression life-cycle the warm sub-tropical air is eventually left “sequestered” as a warm pool trapped in the middle of the mature depression which is called a “warm seclusion”. The usual process of occlusion is bypassed as the centre of the low fills with warm air. Meanwhile, the rapidly overshooting upper cold front causes S-K cyclones to often elongate in appearance on surface pressure charts, a feature associated with the rapid forward acceleration of the cold front in relation to the tightly wrapped, almost stationary, wrapped warm front. It is this tightly wrapped warm front (sometimes shown as occluded on weather charts) that shows another defining feature of S-K depressions.

RaCy Atlantic chart Dec 2014

elongated depression

filling off Norway

As our initial bomb LOW pressure moved due east and filled and decayed offshore near Norway, a wave depression further south on the Polar Front also “bombed-out” to the SW of the UK and swept across Southern England on Thursday-Friday 11-12 Dec.

wave depression

turned RaCy

satellite image Dec 11

diminutive wind impact

squall line cold front

bombing out central pressure, a bit

MetOffice yellow warning

GFSP and GFS forecasts: turned out in between

This was a separate small scale system but technically another rapid cyclogenesis as central pressure fell more than 24mb in 24 hours, but only just. This illustrates the varying scale of bomb cyclones: some cover vast areas, some a small. The 11-12 Dec RaCy depression was much smaller in size and intensity, max wind speeds were much more restricted and the whole system several magnitudes smaller in scale than the “mother” cyclone further north. Charts below show the evolution of this storm.

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Finally, the North Atlantic Oscialltion is a measure used to describe and forecast the mean pressure pattern over the Atlantic. A positive NAO indicates “normal” conditions with low pressure over iceland and high over the Azores. This is associated with a zonal west to east flowing jetstream and fast moving cyclones moving rapidly west to east bringing generally mild conditions to the UK in winter. Note the recent positive pattern matching the westerly flow and active zonal jetstream causing the RaCy depressions. When the NAO turns negative the jetstream is often more wiggly and flows between latitudes in a more meridional flow potentially bringing cold air from the north when pressure patterns are more slow moving and even “blocked”. A negative pattern is often associated with cold winter weather for the UK. The NAO is not a driver of weather, merely an indicator of pressure patterns.

For a bit of fun we invented our own local Wight-Wash Oscillation (WWO) which is a measure of pressure across the south of England between The Wash and the Isle of Wight. This would give an approximately similar local version of the NAO but just for fun! We noted a WWO difference in pressure of 10mb during St Jude and only 9mb during the recent bomb wave depression. The WWO particularly suits the passage of wave depressions across the Midlands which tend to yield the highest wind speeds for the SE. It would also work in negative conditions which would give cold easterly winds in winter. Note this measure is just for fun!

Positive NAO remains likely on the run-up to Christmas 2014 so chances of a White Christmas is much reduced. Remember that a White Christmas for us in SE England is the rare exception to the rule. On a brighter note, the earliest sunset has just passed and we can at least look forward to later sunsets from now on!